3. The super majority required is of the Senators present.

If a case of blue flu broke out among the Republican conference, all that would be required is for the Democrats and enough Republicans be present to make a quorum and then two thirds of those present to vote to convict.

4. Thanks for asking this - I had been wondering the same thing.

5. Senate GOPers missing the vote as a mechanism to convict Trump requires MORE

rather than fewer to be willing to go along with the plan of conviction and removal.

With all Senators present, and 47 Dems, they need 20 GOPers to vote to convict to get 67/100 (67% is just above the 2/3 threshold).

If the GOP "defectors" (i.e. non-traitors-to-the-Constitution-and-their-oaths) instead want to miss the vote as a means of removing Trump with only Dem affirmative votes, because they don't vote against Trump and thereby increase the numerator, you need more participants in that plan to reach the 2/3 threshold. Specifically 30 GOP Senators. 47/70 is 67.14%.

Abstainers are only worth a fraction of convictors, so more are needed -- probably not a viable plan at this scale because of how obviously contrived it looks. If they are going to bail on the Trump-humpers they might as well do so effectively and strongly, and not look like a waffler or a coward.